Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other systems throughout much of the world. Porous ceramic structures, such as, for example, cordierite ceramic structures, are often used in combustion and other systems to filter and remove particulates from fluids, such as, for example, soot and ash from exhaust gas. Such structures are generally prepared using a mixture of raw materials, which include, for example, for cordierite structures, base ceramic materials capable of forming cordierite upon firing (e.g., clay and talc), a binder (e.g., an organic cellulose ether, such as, for example, water-soluble methyl cellulose (methocel) or hydroxypropyl methyl cellulose), and a pore-forming agent (e.g., a starch). The mixture of raw materials is formed into a green structure, which is typically extruded to form a network of channels or cells (sometimes referred to as a honeycomb) configured to flow gas therethrough. The green structure is then fired to form the final ceramic product, such as, for example, cordierite particulate filters.
Such particulate filters may include, but are not limited to, diesel particulate filters that are sometimes classified by their size and the type of vehicle and/or use for which the filters will be utilized. For example, light-duty diesel (LDD) filters are typically 7 inches or less in diameter and are generally used in vehicles with unpredictable engine duty cycles (e.g., hard-duty cycles), such as, for example, passenger cars and light trucks. Heavy-duty (HDD) filters are typically 9 inches or greater in diameter and are generally used in vehicles with predictable engine duty cycles, such as, for example, large, heavy trucks (e.g., semis and/or other commercial hauling trucks).
To gain increased efficiency in making ceramic structures, such as, for example, LDD, HDD, and other types of particulate filters and/or cellular ceramic structures, it is desirable to shorten the time involved with firing the green structure to produce the final ceramic structure. However, in some cases, increasing the temperatures during the firing cycle too quickly leads to undesirable cracks in the final ceramic structure, such as, for example, in the internal portions of the structure. The problem of product survivability (e.g., elimination of cracking) has thus conventionally been dealt with by slowing down the firing cycle. In other words, the firing temperature may be increased to a peak temperature over a relatively large time period so as to avoid heating the green structure too rapidly leading to cracking.
Other methods that have been used to eliminate and/or minimize cracking include firing in a low oxygen environment, altering the raw material composition of the green structure, and/or utilizing firing additives, among others. Such techniques, however, may be product specific, thereby requiring a significant amount of trial and error before yielding desirable results. Some techniques also may be relatively expensive due to the use of relatively expensive materials and/or gas content during firing.
It may be desirable, therefore, to provide a method of firing a green structure to produce a ceramic structure that minimizes the length of the firing cycle, while reducing and/or eliminating cracking, thereby providing increased survivability for a broad range of product types, including, for example, relatively large cellular ceramic structures such as, for example, HDDs. It also may be desirable to provide a method of firing a porous ceramic structure that provides increased product survivability without requiring the addition of expensive gases and/or other firing additives.